Advanced Spectroscopic Characterisation of Metal-Organic Framework Nanosheets and their use in Phosphate Sensing

Understanding the relationship between material structures and optical and electronic properties is key to enabling their use in a wide range of applications. In this thesis an in-depth investigation of a class of porphyrin-based metal organic framework nanosheets (MONs) was undertaken into the effects of nanosheet stacking, aggregation, and metalation on absorption, emission, and excited state dynamics.
Concentration and centrifugation dependent aggregation of Zn2(TCPP), where TCPP is tetra(carboxyphenyl)porphyrin, was identified to cause wavelength-dependent emission quenching and increased exciton annihilation in aggregates, but also increase the excited state lifetime. Additionally, post-synthetic metalation at the porphyrin centre was investigated to introduce selective wavelength emission quenching, reduced exciton annihilation, and longer excited state lifetimes due to enhanced intersystem crossing. Specifically, avoiding aggregation and metalation with any of the metals tested is revealed to decrease the amount of exciton annihilation in these MONs. Variation in the effect different metals had is explained using electronic hybridisation theory and physical porphyrin deformation theory, providing a foundational approach to understanding the effects on these materials. The conclusions drawn from these analyses can advise the fabrication of these materials for specific optoelectronic properties required depending on application.
An ambitious experiment to ‘topographically’ map the emission output of Zn2(TCPP) MONs was also undertaken using a modified surface near-field optical microscopy setup to collect spectroscopic data for the analysis of single nanosheets and allow correlation with particle thickness. This proved beyond the capabilities of the setup during the research timeframe, however, successful imaging of ~8 μm aggregates was achieved and there remains significant remit for optimisation and use with other materials.
Finally, application of MONs and other materials in the sensing of phosphate fertilisers for more sustainable farming were investigated. Both Zn2(TCPP) MONs and molecularly imprinted polymers (MIPs) were investigated to introduce selectivity to carbon electrodes, resulting in enhanced sensitivity and some selectivity toward phosphate over nitrate analytes.